This invention relates to a method of demodulating an angle-modulated signal, comprising quardrature-mixing said angle-modulated signal with a local oscillator signal to produce a pair of signals in each of which a frequency within the bandwidth of the angle-modulated signal has been translated to zero, differentiating with respect to time a signal derived from one signal of the pair and multiplying the result by a signal derived from the other signal of the pair to produce a first auxiliary signal, differentiating with respect to time the signal derived from the other signal of the pair and multiplying the result by the signal derived from the one signal of the pair to produce a second auxiliary signal, forming an output signal proportional to the difference between said first and second auxiliary signals, and varying the proportionality factor in dependence upon the sum of the squares of the instantaneous amplitudes of said signals derived from the one and the other signal of the pair. The invention also relates to a demodulator for performing such a method.
A method of the above kind is known from an article by J. K. Goatcher, M. W. Neale and I. A. W. Vance entitled "Noise considerations in an integrated circuit VHF radio receiver" in the Proceedings of the IERE Clerk Maxwell Commemorative Conference on Radio Receivers and Associated Systems (IERE Proceedings No. 50), University of Leeds, 7th-9th July 1981, pages 49-51.
Demodulating an FM signal by quadrature-mixing it with a local oscillator signal to translate a frequency within its bandwidth to zero, differentiating each member of the resulting pair of signals with respect to time and multiplying the result by the other member of the pair, and subtracting the results of the multiplications once from the other to produce an output signal is known from Patent Specification GB-A-1530602. Unfortunately the output signal obtained by means of this basic method has a square-law dependence on the levels of the signals of the pair. Accordingly, in the method known from the above-mentioned article the two signals of the pair are additionally squared, the results added together, and the signal resulting from this addition is fed to the control input of an amplitude divider which operates on the difference between the auxiliary signals.
Receivers in which an FM signal is quadrature-mixed to translate a frequency within its bandwidth to zero suffer from the disadvantage that the resulting d.c. component in each signal of the pair of signals obtained by means of the mixing operation cannot be distinguished from unwanted d.c. offsets which inevitably exist at the output of each mixer. During reception of the smallest detectable signal these offsets are typically 60dB larger than the wanted d.c. components. Moreover, each signal of the pair has to be amplified by, typically, 90dB, without limiting, before it is processed further. This is impracticable, using circuits operating from reasonable supply voltages, unless the d.c. components (both wanted and unwanted) are eliminated by suitable filtering, for example by the insertion of d.c.-blocking series capacitors or equivalent circuitry (see for example copending UK Patent Application No. 8512491 (PHB 33174)) in the path for each signal of the pair. The cut-off frequency of such a filter cannot be made arbitrarily small, since, at least in the case of portable or mobile receivers, the d.c. signals being eliminated cannot be regarded as time-invariant. Unfortunately the provision of such filters creates a hole in the effective input pass-band of the receiver, and this results in distorted recovery of the original modulation by the method described in the aforesaid patent specification GB-A-1530602, and indeed by any other known method. Employing, in addition, the simple square, add and divide mechanism described in the aforesaid article by Goatcher, Neale and Vance is found to make matters subjectively even worse, at least when the modulation is speech.